1. Field of the Invention
The present invention relates to a temperature control material formed to control the temperature of an object of control using a temperature indicator which changes its color by temperature and a temperature control method using the same.
2. Description of the Prior Art
Out of conventionally used temperature indicators, temperature indicators which respond to temperature to change their colors include thermochromic organic dyes such as ethylene derivatives obtained by substituting the condensate aromatic ring of a spiropirane, bianthrone or dixanthylene, a metallic complex salt crystal made of CoCl2.2(CH2)6N4.10H2O and a combination of an electron donor color developing compound, electron acceptor compound and a polar organic compound. The temperature of a temperature control label or the like made from any one of these temperature indicators is judged by observing its color change by temperature visually.
Temperature indicators are roughly divided into reversible and irreversible types. The reversible type changes its color each time temperature varies and the irreversible type changes its color at a certain temperature and does not return to its original color.
The reversible type is used to tell the time to have a drink such as beer or to arouse attention visually.
The irreversible type is used to control the temperature of a vegetable, fresh food or the like and described in Examined Japanese Patent Publication No. sho 58-10709 as a temperature indicator which records temperature history. That is, ink whose viscosity changes according to temperature (for example, ink containing a dye dissolved in oleyl alcohol) permeates into a penetrant and how long the irreversible temperature indicator has been exposed to a temperature higher than a set temperature can be detected from the length of ink permeation. There is also available an irreversible temperature indicator which comprises a plurality of layers over the label, that is, an ink layer, separate layer, porous layer and display section. The separate layer is removed when temperature control is started, and the porous layer is provided to control the ink permeation time of the ink layer. When the set temperatures of these layers are lower than room temperature, storing means is required when temperature control is not started to effect low-temperature irreversibility. Thus, the irreversible temperature indicator is expensive and has a complex structure.
Unexamined Japanese Patent Publication No. Hei 8-197853 discloses an alarm method which makes use of the fact that the dispersion speed of molecules greatly changes when the temperature exceeds the glass transition temperature as the set temperature of a temperature indicator.
The problems of these prior arts will be described below. The present invention relates to an irreversible temperature indicator whose set temperature is lower than environmental temperature. A temperature indicator disclosed by Examined Japanese Patent Publication No. Sho 58-10709 requires storing means when temperature control is not carried out and hence, is expensive and complex in structure, and a temperature indicator disclosed by Unexamined Japanese Patent Publication No. Hei 8-197853 merely gives an alarm that the temperature of an article of commerce to be controlled exceeds the set temperature (glass transition temperature) by a quick change in the density of its developed color and cannot monitor which environmental temperature the article is kept for a long time. That is, when an article of commerce which must be maintained at a low temperature is delivered to a shop in a chill car and transferred to a refrigerator at a shop from the chill car, the density of a color developed by a temperature indicator changes upon exposure to the outside temperature as the glass transition temperature of the temperature indicator is generally lower than normal temperature. Therefore, the temperature indicator cannot be used for the subsequent temperature control any longer.
It is an object of the present invention to obtain a temperature control material which can carry out temperature control without hindrance after the temperature exceeds control temperature temporarily and a temperature control method using the same.
The object of the present invention is achieved by the novel temperature control material and the temperature control method using the same of the present invention.
According to one aspect of the present invention, there is provided a temperature control material comprising a temperature indicator whose developed color density differs according to temperature, which is irreversible at environmental temperature, which changes its color according to crystal or non-crystal, or phase separation or non-phase separation and whose glass transition temperature is set to a temperature higher than control temperature. The temperature indicator may contain a binder resin.
According to another aspect of the present invention, there is provided a temperature control method using a temperature indicator whose developed color density differs according to temperature, which is irreversible at environmental temperature and which changes its color according to crystal or non-crystal, or phase separation or non-phase separation. That is, the method of the present invention comprises the step of setting the glass transition temperature of the temperature indicator at a temperature higher than control temperature and the step of carrying out temperature control by irradiating the temperature indicator with light having a wavelength which is absorbed by a color developed by the temperature indicator and detecting the intensity of the reflected light or transmitted light. It is possible to give an alarm that the temperature indicator is exposed to a temperature higher than its glass transition temperature with its color development in a short period of time.
According to still another aspect of the present invention, temperature control is carried out by attaching a temperature control material comprising a temperature indicator which is distributed at a temperature lower than the outside temperature, whose developed color density differs according to temperature for an object of control which is exposed to the environment of the outside temperature for several minutes in the distribution step, which is irreversible at environmental temperature and which changes its color according to crystal or non-crystal, or phase separation or non-phase separation and whose glass transition temperature is set to a temperature higher than control temperature, by irradiating the temperature indicator with light having a wavelength absorbed by a color developed by the temperature indicator, and by detecting the intensity of the reflected light or transmitted light.